1. Field of the Invention
The present invention relates to an intermediate shaft that is incorporated in steering systems of automobiles.
2. Description of the Related Art
For example, an intermediate shaft is formed by coupling an inner shaft and a tubular outer shaft by spline fitting so that the inner shaft and the outer shaft can slide relative to each other in the axial direction (in a telescopic manner). The outer peripheral surface of external splines formed on the outer periphery of the inner shaft or the inner peripheral surface of internal splines formed on the inner periphery of the outer shaft is typically coated with a resin coating layer in order to substantially eliminate clearance between the external and internal splines to reduce rattling noise or to reduce backlash of a steering wheel during steering operation (see, e.g., Japanese Patent Application Publication No. 2012-117560 (JP 2012-117560 A)).
Polyamide resins having high mechanical strength and excellent self-lubricating properties, etc. are commonly used as a base resin of the resin coating layer.
In recent years, there has been a demand for an intermediate shaft that is conventionally disposed in passenger compartments of automobiles to be disposed in engine compartments etc. where the ambient temperature can become high. The resin coating layer is therefore required to have higher heat resistance. As the resin expands with an increase in temperature, the pressure on the contact surface of the resin coating layer increases accordingly. A sliding load therefore tends to increase with an increase in temperature. Accordingly, it is particularly required to further reduce friction under the high temperature conditions.
Moreover, the resin coating layer is required to maintain satisfactory sliding properties for a period as long as possible particularly in a wide range of ambient temperatures from around normal temperature before starting of an engine to a high temperature during driving of the engine (about 140° C.). However, conventional resin coating layers made of only a base resin such as a polyamide resin are not suitable for use in the engine compartments because their sliding properties are significantly degraded in a high temperature range of 120° C. or more and such degradation in sliding properties results in increased wear.